The circuit breakers of today that are being provided in electrical power lines of power supply systems are not only provided with a circuit breaking element, i.e. an element that disconnects the power line from a power source, they are also provided with fault detecting abilities. They thus detect a fault on a power line and then open the circuit breaking element.
Circuit breakers are furthermore often equipped with a re-closing functionality. This means that after opening the circuit breaking element upon a detection of a fault, like for instance a low impedance, the circuit breaking element is again closed after a pre-determined disconnection time interval has lapsed, where the length of this time interval is dependent on the frequency used in the supply of power. In the case of a power supply frequency of 50 Hz it is usually around half a second long. If then the fault remains, i.e. is a permanent fault, the circuit breaking element is again opened and thereafter kept open until the fault has been removed. Such fault removal is normally performed by service personnel and may therefore take considerable time. The reason for such re-closing is that many faults are temporary, i.e. of a transient nature, and will disappear after some time. This may for instance be the case with lightning striking a power line. In order to not to keep the power down for too long it is then advantageous if the circuit breaking element is re-closed so that power supply can be resumed if the fault no longer exists.
WO 2007/114566 gives an example of a circuit breaker, which can be used for such normal circuit breaker operation.
However, if the fault is a permanent fault, like for instance if the power line has fallen on the ground, the re-closing would just lead to an immediate opening again. To open and close a circuit breaking element in the case of such permanent faults are strenuous for the equipment in a power supply system. They are normally not dimensioned for prolonged or repeated exposure to current and/or voltage levels that exist during such permanent faults, especially if the fault is a grounding fault. They may thus break down because of such faults. Such a permanent fault can also have a very severe impact on the power supply system. A fault may spread over a large area of the power supply system as a sudden depression of voltage or voltage sag which might trip sensitive devices. The efforts needed to reinstate the devices and the power network, are of serious concern. Also the circuit breaker may break down.
It would therefore be of interest not to perform re-closing in a circuit breaker in case of a permanent fault but still use the re-closing functionality for temporary faults.
It is at the same time hard to determine if a fault is temporary or permanent when the circuit breaking element is open, since the current and voltage of the power line are normally used in determining these types of fault. When the circuit breaker has broken the connection with the power source it is then not possible to measure these currents and voltages.
DE 3429150 describes the principle of injecting a signal having an audio frequency into a power line for determining that a safety ground does not remain before this power line is put into use. The earthing of the power line is here checked through measuring the power of the signal returned by the power line.
It would therefore be of advantage to provide an improved re-closing functionality of a circuit breaker, where it is possible to separate a temporary fault from a permanent fault in order not to put unnecessary strain on the equipment used in power supply systems.